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In Rio, at her first Olympics, McCartney performed with poise, a calmness that belied her 19 years. In a moment she became the new golden girl of New Zealand athletics.

She took it all in her stride. No fuss, no pressure.

Little more than a month later the teenager, who is studying part-time for a Bachelor of Science in physiology, remains unaffected by her meteoric rise.

"All I want to do is reach my potential. I think when people said I looked calm at the Olympics, it was because I was worried about what I was jumping and not how everyone else was jumping. I wasn't worried about the result - I think that's what got me the medal."

But McCartney's road to Rio was almost a cul-de-sac. Behind the Colgate-white smile and infectious laughter there were problems - mental blocks that stopped her training, destroyed her confidence and ability to perform at the highest level.

The first six months of last year were a write-off. Even that far out from Rio her long-term training programme should have been well under way.

"It was a really tough and frustrating time," she says. "It's not so much depression but an athlete's mental block, like a type of mental illness. You want to snap out of it but you can't. I didn't know how to at that point, it's much harder than an injury to deal with.

"The thing with pole vault is if you are going through a hard mental block, you can't even take off into the air. You can't do anything. You can't compete - it's a bit scary.

Scary but not rare. Leading sports psychologist Karen Nimmo says it's very common for athletes to experience mental blocks during training or performance.

"Every athlete goes through these kinds of challenges at some point," says Nimmo. "The first time you experience one it can be quite daunting because you don't know what's going on, if it's normal or how best to go about correcting them."

McCartney credits her "tight team" - her family, coach, physio and doctor - for getting her over the struggle to perform. Her parents kept a healthy distance, offering a constantly reassuring presence.

"They never pushed me and said 'You'll be fine, you'll get through it'. They didn't have to say anything, they were just there. The thing is you have to do it yourself - you have to get yourself out of there."

So she did. The tenacious teenager and her coach Jeremy McColl started from scratch.

"I went back to the basics of training and slowly built it back to where my confidence was. Instead of trying to push on and on, we took a step back and I went back to enjoying training without the stress."

Her loving family background must have helped.

McCartney spent the first 10 years of her life in Ponsonby before the family moved to Devonport. She still lives at home with mum Donna Marshall, a city GP, dad William McCartney, an accomplished Auckland barrister, and her two brothers, Finn, 18, and Hamish, 16.

Although her public profile has increased, her family, home and boyfriend are all "off limits". A request to conduct this interview at the family home was, predictably, politely declined, and instead the interview took place at an aunt's house in Herne Bay.

"We are like everyone else, nothing too exciting - I am no one special," McCartney says.

She says she's inherited her mother's work ethic and organisational skills, her dad's "inherently happy laugh".

The most important thing her parents have taught her is to "enjoy myself and be comfortable with whatever I do. Don't be forced into something you don't want to do."

Her love of sport began at Ponsonby Primary.

"I was fairly adventurous, I was almost into every sport there was," she says, reeling off cross country, netball, high jump, basketball, touch rugby, ripper rugby, actual rugby, squash, tennis, water polo, swimming and jazz ballet.

"Those the only ones I can think of right now," she laughs.

"Netball was my favourite," she says.

"There is something about running around and jumping as high as you can. There is nothing better than intercepting a ball - it feels so good.

"I had dreamed of being a Silver Fern one day and was gutted to drop netball for pole vault."

The latter began simply as something to have a go at. McCartney first picked up a pole aged 14, when a friend introduced her to the decathlon.

"It wasn't my plan to be an Olympic pole vaulter, I just wanted to try it out, so I kept going on with it. You don't love it every single day but when it goes right, you can't not love it."

It went right in Rio. When she arrived she felt relaxed with "nothing to lose".

Despite her travails of the previous year, she felt no pressure to win a medal. That was her goal for the 2020 Games in Tokyo.

"It was my first Olympics. I was just focused on what I was doing. But once I got into the finals, I thought, 'Wow, I have to keep fighting, I can't let this slip away, it's actually in my grasp now so I have to keep pushing for it.'"

The underdog equalled her personal best height of 4.8m to earn third place and the bronze medal on a countback from Boyd.

McCartney was buoyed by hearing her parents, brothers, boyfriend and two aunties cheering her on in the stands.

"You can figure out the people's voices you know really well. When I looked into the crowd, all I remember seeing was a group of people all wearing black, and that's awesome when you are a Kiwi overseas - you immediately think of home."

The night before the final, friend and mentor Sarah Walker kept her cellphone by her bed in case McCartney called for "last-minute advice".

Walker knows pressure. She won silver in the BMX at the 2012 London Olympics.

Injury prevented her from competing in Rio but the pair were reunited when Walker was elected to the International Olympic Committee's athletes' commission.

McCartney says she will always cherish the moment she knew she'd "nailed it" and vaulted 4.80m, higher than a double decker bus and the Berlin Wall.

"When you get the timing right you can actually feel the energy of the pole bending and throwing you upwards and you know you've done it right and you go flying over the bar.

"Before I land on the mat, it's so satisfying to see the bar is still sitting up and it hasn't come down, so I am often celebrating on the way down."

She says the medal ceremony was overwhelming, because she was exhausted and emotional.

"I was a mess. When I was at the podium and saw my family and support crew, and the New Zealand flag - I felt so emotional I started to cry."

And she was surprised to learn she had become the youngest Kiwi woman to win an Olympic medal.

"The hardest thing to get your head around is knowing you are the first - not everyone is the first to do something."

The moment was short-lived. Lydia Ko, younger by about four months, won a medal less than 24 hours later.

"That's okay, she got a silver anyway - better than a bronze", said McCartney graciously.

"It doesn't matter because we are so close in age anyway, but the fact that two Kiwi athletes could do that at an Olympic Games is mind-blowing."

Like McCartney, Ko spent key formative years on the North Shore. Lorde completes a trio of young female world-beaters from across the bridge.

The Grammy-winning pop star - real name Ella Yelich-O'Connor - sent her childhood friend a tweet after watching her win bronze: "AHHHHHH CRUSHED IT ELIZA".

"I don't have Twitter," says McCartney. "That was funny - I haven't seen Ella for years because she's doing incredible things. It's cool she is still linked into New Zealand. It's great we [Lorde, Ko and McCartney] are all from the North Shore. It's a very small world. But I think that's what makes it special."

Lorde completes a trio of young female world-beaters from across the bridge.

After praise from pop stars and the support of a nation, McCartney's life is back to what passes for normal now.

That equates to intensive training six days a week. Sunday is her only day off, when she can sleep in and "hang out with her friends".

"I am good at watching TV [favourite shows include Breaking Bad and Game of Thrones] but relaxing isn't something I take for granted because it doesn't happen very often. When I'm sitting down and can't think of anything to do, it's concerning because it's like 'What have I forgotten to do?'."

The Weekend Herald interview was on a Sunday. McCartney talked of how she'd become a "business". She said she had to teach herself how to do GST returns.

A schedule like that wouldn't leave much time for romance. McCartney is coy about her boyfriend, Kiwi kiteboarding champ Lukas Walton-Keim.

She says they've been together for 18 months and the relationship means a lot to her. Lukas was by her side in Rio.

By her own admission, McCartney is a "goody-good". She can't remember the last time, if ever, she's had a hangover, but confesses to a "terrible sweet tooth" and eating "crap food" when not training.

"I can eat a whole [kingsize] bar of chocolate in one go and cake for breakfast."

As she notes, she's entered the commercial world. Her new-found fame means sponsorships and media requests. McCartney is sponsored by Nike and is a face of Anchor Milk. This week she joined other "iron maidens" Sarah Walker, Sophie Pascoe and Lisa Carrington in the new Beef and Lamb campaign.

She's had to get used to "people coming up to me at the supermarket" and the "weird feeling" of seeing herself on gigantic billboards.

She's philosophical about the sacrifices she's had to make, which include missing out on studying at Otago University, where her parents went, "living in a hall of residence and just being a student".

"Even when I'm at [the University of Auckland where she studies physiology] I just go in and out to my lectures. It's not like there's any student life I get to be a part of, so I guess I miss out on things. It's definitely a sacrifice but I have goals that I am working towards so that makes it okay. Day-to-day you can't do things other people are doing but I am okay with that, I know it will pay off in the end."

One payoff would be another medal in Tokyo. Another would be a world record, although she admits that "could be years away".

"I think it can be pushed, so that's the next thing big thing, and it's a pretty big thing to keep you motivated.

"The funny thing about sport is that it's very rarely so rewarding, but when it is it's just the best feeling.

"In Rio it was great to know your country and everyone in it is behind you. We are a small but strong sporting nation - you really feel that, that's what's special."